HERE COMES A BIOLOGY NERD :D ...... who actually had to look up the answer to a side question in the episode, since arthropods aren't one of my major interests! x3;

Truth be told, the difference between insects and crustaceans is pretty nuanced -- but it's not just that "one lives in the water and the other doesn't."

Insects, crustaceans, myriapods (like centipedes and millipedes), and arachnids are all arthropods ("jointed feet"), members of a super diverse phylum of animals which all share three main characteristics; an exoskeleton, pairs of jointed legs/appendages, and segmented bodies. Insects are a very specific type of arthropod -- even though they're the most diverse group of animals on the planet. Crustaceans, less so. Crustaceans are like the "miscellaneous" class of arthropods. :B

To be an insect, an arthropod needs...

A three-segmented body -- head, thorax, abdomen.

Three pairs of legs -- no more, no less.

Compound eyes.

One pair of antennae.

If you don't have all of those, you're not an insect. But within those classifications? Go wild. Kind of amazing there are more than a million species of insect, isn't it? :3 Also, general guideline? If it's an arthropod and has wings, it's an insect. The other groups don't have 'em.

Crustaceans' membership guidelines are a little looser. Their main characteristic, aside from the general arthropod ones, is that if they have antennae, they have two pairs of them.

That's it. :B

Crustaceans are more defined by what they DON'T have as opposed to what they DO have. Like I said, it's kind of a catch-all category -- the easiest way to tell if you're an arthropod person is to check out their mouths. Insects and myriapods have well-developed mouthparts; arachnids have chelicerae, which are basically specialized "limbs" that act as little nippers and nibblers to help break up prey and/or inject venom. (Spider fangs are chelicerae. Scorpion pincers are not; the chelicerae are tiny things near the mouth. The pincers are pedipalps, which are basically specialized appendages to help grasp and move food to the chelicerae. Spiders have them, too. But obviously differently-shaped and smaller. :B) Crustaceans tend to either be filter feeders, or have sets of mandibles/maxilla that are somewhere between insect mouths and chelicerae.

You were close in one respect, though! The vast majority of crustaceans are aquatic, while the vast majority of insects aren't. :3 There are a few exceptions, though -- like woodlice/pillbugs, a type of cute little garbage-muncher that lives in moist places and chows down on decaying organic matter. Those are crustaceans. And there are aquatic insects, too, like whirligig beetles -- plus some insects, like mosquitoes (blech), lay eggs and live their early lives in the water.

Basically you guys were close, but not quite there -- and apparently you need to study arthropods pretty closely to be able to really tell the difference yourself. XP So good on ya! =w=d